1. Field of the Invention
The present invention relates to image processing. More specifically, a system for capturing and processing video images.
2. Background
Digital cameras are used in a variety of applications requiring image capture and image processing. Many applications require cameras which are economical, yet generate a high quality video signal. Typical solid state sensors used in digital cameras have a dynamic range (or light intensity range) of 1000:1 or greater. However, many existing digital cameras utilize an inexpensive 8 bit or 6 bit analog-to-digital (A/D) converter to generate a digital output signal representing the captured image. The use of an 8 bit A/D converter limits the dynamic range of the output signal by providing a maximum of 256 possible luminance levels. Therefore, the dynamic range capabilities of the sensor (1000:1) are compressed to 256:1 by the A/D converter. This reduction in dynamic range results in “clipping” of the image; i.e., loss of image detail in bright areas and dark areas of the image. When clipping occurs, dark areas of the image become black (e.g., luminance level 0) and bright areas of the image become white (e.g., luminance level 255).
Existing cameras attempt to compensate for this dynamic range reduction by using an automatic gain control (AGC) amplifier having different gain settings. The gain setting of the AGC amplifier is determined based on the total luminance entering the camera. Since a single luminance level is determined, the selected gain setting is applied uniformly to the entire image.
An example of an existing camera is illustrated in FIG. 1. A digital camera 10 is attached to an image processor 12. Camera 10 captures an image of scene 14 and generates a digital signal representing the captured image. A solid state sensor 16 in camera 10 captures an image of scene 14. Typical solid state camera sensors are capable of discriminating light intensity over a dynamic range of 1000:1 or greater. Sensor 16 generates a sensor output signal representing the sensed image and provides the signal to an AGC amplifier 18. AGC amplifier 18 applies a particular gain setting to the sensor output signal. A/D converter 20 receives an output signal from AGC amplifier 18. As discussed above, A/D converter 20 may be an 8 bit or 6 bit converter. Using an 8 bit A/D converter 20, the dynamic range of the sensor output signal is compressed to 256:1. Similarly, using a 6 bit A/D converter 20, the dynamic range of the sensor output signal is compressed to 64:1. This dynamic range compression results in a loss of image detail at both ends of the dynamic range; i.e., bright areas and dark areas. As a result, bright areas of the image are compressed into all white regions and dark areas of the image are compressed into all black regions.
AGC amplifier 18 typically has several different gain settings which may be applied to the sensor output signal. The required gain setting for a particular image is selected based on the total light entering camera 10. If the total light level is low, the gain setting is increased. Similarly, if the total light level is high, the gain setting is decreased. The particular gain setting selected is applied uniformly to the entire image.
A/D converter 20 generates a digital video output signal on a signal line 22. As discussed above, the dynamic range of the digital video output signal is limited by the capacity of A/D converter 22.
FIG. 1 also illustrates image processor 12 coupled to camera 10. Image processor 12 is an optional component, and is not required for proper operation of camera 10. The digital video signal output provided on line 22 may be connected directly to a device capable of accepting digital signals. The optional image processor 12 includes a video processing circuit 24 and a digital-to-analog (D/A) converter 26. Video processing circuit 24 receives the digital video signal output from line 22 and performs various processing of the signal. Various types of video processing circuits and video processing functions will be known to those skilled in the art. D/A converter 26 receives a signal from the video processing circuit, converts the signal to an analog value, and generates an analog video signal output on signal line 28. This analog video signal may be transmitted to any device capable of receiving analog video signals.
Problems associated with dynamic range reduction may be solved by utilizing a 10 bit or 12 bit A/D converter to preserve the dynamic range of the sensor output signal. However, 10 bit and 12 bit A/ D converters are expensive and substantially increase the cost of the camera. In an application requiring an inexpensive camera, the use of 10 bit or 12 bit AID converters is not practical.
It is therefore desirable to provide a low-Cost digital camera utilizing an inexpensive A/D converter, yet capable of generating a video signal containing image detail in bright areas and dark areas of the image.